The present invention relates to a method and apparatus for introducing gaseous fuel into the cylinder of an internal combustion engine. More specifically, the present invention relates to a method of, and apparatus for, two-stage injection of gaseous fuel into the engine""s cylinder to control the combustion mode of the gaseous fuel introduced in the two stages.
The internal combustion engine industry is under ever increasing pressure to reduce pollution to the environment by lowering harmful engine emissions. One response to this pressure has resulted in research into adapting compression ignition (CI) engines (also known as xe2x80x9cdieselxe2x80x9d engines) to burn natural gas instead of diesel fuel. Compared to diesel fuel, natural gas is a relatively clean burning fuel and the substitution of natural gas for diesel fuel can reduce emission levels of both nitrogen oxides (NOx) and particulate matter (PM). A known technique for substituting natural gas for diesel fuel is called dual fuel operation. In this method, natural gas is mixed with intake air prior to introducing the air/natural gas mixture into the engine cylinder (a process known in the art as fumigation). The mixture is then introduced into the piston cylinder during the intake stroke. During the compression stroke, the pressure and temperature of the mixture are increased. Near the end of the compression stroke, dual fuel engines inject a small quantity of pilot diesel fuel to ignite the mixture of air and natural gas. Combustion is triggered by the auto-ignition of the diesel fuel and it is believed that a propagation combustion mode occurs under these conditions. One advantage of employing a pre-mixed charge of air and natural gas is that the fuel to air ratio can be lean. With fumigation it is possible to realize the advantages of xe2x80x9clean burnxe2x80x9d operation, which include lower NOx emissions, lower PM and a potentially higher cycle efficiency.
Known dual fuel methods, however, have at least two disadvantages. One disadvantage is encountered at high load engine operating conditions, when the elevated temperature and pressure in the piston cylinder during the compression stroke makes the air/natural gas mixture susceptible to xe2x80x9cknockingxe2x80x9d. Knocking is the uncontrolled auto-ignition of a premixed fuel/air charge. Knocking leads to a rapid rate of fuel energy release that can damage engines. Measures to reduce the risk of knocking include lowering the compression ratio of the piston stroke or limiting the power and torque output. These measures, however, cause a corresponding reduction in the engine""s cycle efficiency (that is, not as much power is available from each piston stroke).
A second disadvantage of known dual fuel methods is that under low load engine operating conditions, the mixture of fuel and air becomes too lean to support stable premixed combustion and results in incomplete combustion or misfiring. The intake air flow can be throttled to maintain a mixture that can sustain premixed combustion, but throttling adversely affects engine efficiency.
A different type of engine that is under development for substituting gaseous fuel for diesel fuel in an internal combustion engine is sometimes referred to as a xe2x80x9chigh pressure direct injectionxe2x80x9d (HPDI) engine. Similar to conventional dual fuel engines, which employ the above-described method, HPDI engines burn a large quantity of gaseous fuel, yielding an improvement (over engines burning only diesel fuel) with respect to the emission levels of NOx and PM. In addition, HPDI engines have the potential to achieve the same cycle efficiency, power and torque output as equivalent conventional diesel-fuelled engines. The operational principle underlying HPDI engines is that two fuels are injected under pressure into the engine cylinder near the end of the compression stroke. According to one method, a small quantity of xe2x80x9cpilot fuelxe2x80x9d (typically diesel) is injected into the cylinder immediately followed by a more substantial quantity of gaseous fuel. The pilot fuel readily ignites at the pressure and temperature within the cylinder at the end of the compression stroke, and the combustion of the pilot fuel initiates the combustion of the gaseous fuel. Accordingly, HPDI engines have little or no pre-mixture of gaseous fuel and air, and thus the gaseous fuel burns in a xe2x80x9cdiffusionxe2x80x9d combustion mode, rather than a premixed combustion mode. In a diffusion combustion mode, the bulk of the combustion is believed to occur in a local near stoichiometric reaction zone. The temperature and resulting NOx formation in a stoichiometric reaction zone are higher than the temperature and resulting NOx formation caused by a lean burn premixed combustion mode. An advantage of HPDI engines over conventional dual fuel mode operation is that they are not susceptible to knocking under high load conditions because the air and gaseous fuel are not pre-mixed and the gaseous fuel is not introduced into the cylinder until after the pilot fuel. Another advantage of HPDI engines is the ability to operate under low load conditions without the need to throttle the engine.
Recently, homogeneous charge compression ignition (HCCI) has been considered as an alternative to the propagation mode of combustion for providing a mode of lean burn pre-mixed combustion. Experimental HCCI engines generally introduce a homogeneous mixture of fuel and air into the engine cylinder(s). Under certain conditions, compression heating of the charge leads to ignition throughout the bulk of the pre-mixed charge without any flame propagation, and this combustion mode is defined herein as HCCI. HCCI is essentially a xe2x80x9ccontrolled knockxe2x80x9d condition where the combustion rate is mainly controlled by the chemical reaction kinetics. HCCI is thus distinct from a combustion mode controlled by flame propagation. In a flame propagation combustion mode, when a homogeneous mixture of fuel and air is sufficiently rich to sustain a flame and is ignited at a point, a flame front forms and advances from the ignition point. In a flame propagation combustion mode, the rate of combustion is limited by the transfer of the unburned mixture of fuel and air mixture into the flame reaction zone rather than by the chemical reaction rates.
An advantage of a HCCI combustion mode is that very lean mixtures of fuel and air mixtures can be burned. For example, a fuel/air equivalence ratio of between 0.1 to 0.5 can burn in a HCCI combustion mode, whereas under the same conditions, in a propagation combustion mode combustion would be unstable, leading to misfire or partial burn. With a HCCI combustion mode, under very lean conditions, NOx formation rates can be substantially reduced. Another advantage of a HCCI combustion mode is the potential for the engine to achieve higher cycle efficiencies (relative to a conventional diesel-fuelled engine). With a HCCI engine the rate of combustion is potentially very rapid, resulting in an engine cycle that more closely resembles an ideal cycle. However, a disadvantage of HCCI combustion is the lack of direct control over the start and rate of combustion because only indirect control methods are available. Another disadvantage of HCCI combustion is that at high load conditions, the higher fuel/air ratios result in HCCI combustion rates which may cause engine damage by combusting too rapidly, or by the rate of combustion causing very high in-cylinder pressures. Yet another problem with HCCI engines is the relatively high emissions of unburned hydrocarbons and carbon monoxide.
Because a HCCI combustion mode has the potential to yield substantial reductions in NOx and PM emissions, HCCI combustion modes have been the subject of recent studies and published papers. These publications show that the main control strategies over HCCI mode combustion include: (i) using variable intake manifold temperatures (exhaust gas recirculation (EGR) and intake air heating); (ii) using residual gas trapping; (iii) controlling intake manifold pressure; (iv) controlling premixed charge fuel/air equivalence ratio; (v) controlling fuel type and blend; and (vi) using a variable compression ratio. Extending the operable range for HCCI combustion has been achieved through supercharging, use of EGR to reduce rate of heat release, late injection of diesel fuel, and varying compression ratio. However, none of the investigations into HCCI engine operation have considered the benefits of direct injection of gaseous fuel near top dead centre of the compression stroke, resulting in two separate combustion modes in the same engine cycle.
A method of introducing gaseous fuel into a cylinder of an operating internal combustion engine, which has a piston disposed within the cylinder, the method comprising:
(a) monitoring a set of engine parameters;
(b) determining engine load and engine speed from the set of engine parameters;
(c) in a first stage, introducing a first gaseous fuel into the cylinder where the first gaseous fuel forms a substantially homogeneous mixture comprising the first gaseous fuel and intake air prior to combustion; and
(d) in a second stage, occurring sequentially after the first stage, introducing a second gaseous fuel into the cylinder;
wherein the first and second gaseous fuel quantity is controllable in response to at least one of engine load and engine speed, and at least one of initiation and duration of at least one of the first and second stages is variable in response to at least one of engine load and engine speed. The second stage is preferably initiated when the piston is at or near top dead center.
In a preferred method, within the same engine cycle, the first gaseous fuel combusts according to a pre-mixed combustion mode, and the second gaseous fuel combusts substantially according to a diffusion combustion mode. For improved efficiency and reduced emissions the pre-mixed combustion mode is preferably a homogeneous charge compression ignition mode.
The engine may be a two-stroke engine but is preferably a four-stroke engine to reduce scavenging losses of the air/fuel mixture.
The first stage is initiated so that the first gaseous fuel has time to mix with the intake air to form a homogeneous charge. For example, when the first stage begins during the intake stroke, the first gaseous fuel may be introduced directly into the engine cylinder or into the intake port so that it enters the cylinder with the intake air. When the first gaseous fuel is introduced during the intake stroke, it is preferable for the first stage to begin early in this stroke, for example, at the very beginning of the intake stroke when the piston is at or near top dead centre, to give the first gaseous fuel more time to mix with the intake air. In another preferred method the first gaseous fuel is pre-mixed with intake air prior to being introduced into the cylinder. For example, the first gaseous fuel may be pre-mixed with intake air upstream from a turbocharger or a supercharger.
The set of engine parameters preferably comprises at least one of engine speed, engine throttle position, intake manifold temperature, intake manifold pressure, exhaust gas recirculation flow rate and temperature, air flow into the cylinder, compression ratio, intake and exhaust valve timing and the presence or absence of knocking within the cylinder. Engine speed can be measured directly and is a parameter that is used, for example, to control first and second stage timing. Generally timing is advanced as engine speed increases. Engine throttle position is an indication of engine load, which may be used to control the quantity of the first and second gaseous fuel. Other parameters may be monitored as indicators of the in-cylinder conditions that is preferably controlled to be conducive to combusting the first stage fuel in a HCCI combustion mode.
The timing and fuel quantity of the second stage can be manipulated to influence the in-cylinder conditions in subsequent engine cycles. For example, at least one of second stage gaseous fuel quantity, second stage initiation and/or second stage duration can be varied in response to changes in the value of at least one parameter of the set of engine parameters, to maintain in-cylinder conditions that are conducive to HCCI combustion of the first gaseous fuel. Control of the second stage initiation and/or duration and/or fuel quantity is preferably employed as an additional means for controlling in-cylinder conditions, which may be used in conjunction with more conventional control means such as controlling EGR flow rate or intake air/fuel equivalence ratio. An electronic control unit preferably controls the initiation, duration and quantity of the second gaseous fuel, with reference to a look-up table to determine a plurality of control settings for a given engine load and speed condition.
The second gaseous fuel is preferably employed to supplement the first gaseous fuel when the quantity of first gaseous fuel is knock-limited. This allows the engine to operate at higher load conditions. Accordingly, the quantity of the second gaseous fuel is variable and the quantity increases when the engine load increases.
For turbocharged engines the intake manifold pressure is influenced by exhaust gas pressure and temperature since the exhaust gas drives the turbocharger. Therefore, intake manifold pressure can be controlled, at least in part, by controlling at least one of (a) the quantity of the second gaseous fuel and (b) the time the second gaseous fuel is introduced into the cylinder, since these variables are controllable to change exhaust gas pressure and temperature. For example, when knocking is detected, if intake manifold pressure is increased without increasing first gaseous fuel flow rate, the first stage charge will be leaner and less likely to result in knocking. Accordingly, the initiation, duration and fuel quantity of the second stage can be manipulated to increase intake manifold pressure to reduce intake charge equivalence ratio in subsequent engine cycles when knocking is detected.
For engines that are turbocharged and/or that use exhaust gas recirculation, the exhaust gas temperature has an effect on intake manifold temperature. Accordingly, intake manifold temperature can be influenced in subsequent engine cycles by controlling at least one of:
(a) the quantity of the second gaseous fuel; and
(b) the time the second gaseous fuel is introduced into the cylinder.
Intake manifold temperature has a significant effect on knocking and HCCI combustion. For example, when knocking is detected, a countermeasure to knocking is reducing intake manifold temperature. The second stage initiation, duration and fuel quantity may be used to control intake manifold temperature in conjunction with conventional temperature means, such as, for example, intercoolers and aftercoolers.
In a preferred method the second stage comprises a plurality of fuel injection pulses. A plurality of injection pulses or a shaped injection pulse adds more flexibility. For example, the initiation and/or duration and/or fuel quantity for one pulse can be controlled in response to engine load, and the initiation and/or duration and/or quantity of another pulse can be controlled to influence intake manifold temperature and/or pressure in subsequent engine cycles. That is, the portion of second gaseous fuel that is introduced in a first injection pulse can be increased in response to an increase in engine load. Further, the initiation and/or duration and/or fuel quantity in a second injection pulse is controllable to influence at least one of intake manifold temperature and intake manifold pressure, whereby second injection pulse timing is advanced to reduce intake manifold temperature and/or pressure, and/or fuel quantity is reduced to reduce intake manifold temperature and/or pressure. Intake manifold temperature is preferably reduced when knocking is detected.
The initiation and/or duration and/or fuel quantity for different injection pulses may be independently controlled and at least one of the first and second injection pulses is controlled in response to at least one of engine load and speed. Preferably, the total amount of fuel introduced in the second stage may be determined by the engine load, but the electronic control unit (ECU) may refer to a look-up table to apportion this total amount of fuel between a plurality of injection pulses, with the ECU accounting for the fuel conversion efficiency that corresponds to the timing of the injection pulses.
The method may further comprise introducing a pilot fuel into the cylinder so that it ignites when the piston is at or near top dead centre of the compression stroke. To reduce NOx emissions the pilot fuel injection timing and pilot fuel quantity is controlled to form a substantially lean stratified charge prior to the ignition of the pilot fuel. To form a substantially lean stratified charge, the pilot fuel is preferably introduced into the cylinder when the piston is between 120 and 20 crank angle degrees before top dead center. The timing and amount of gaseous fuel and pilot fuel introduced into the cylinder is preferably electronically controlled.
In addition to other measures that may be taken when knocking is detected, pilot fuel quantity and timing may also be varied when knocking is detected. Whether pilot fuel timing is advanced or delayed in response to detected knocking depends upon several variables, but the ECU preferably determines the appropriate action by referring to a look-up table. Some of these variables include, for example, the current pilot fuel injection initiation, fuel injection duration, engine speed, and current intake manifold temperature and pressure.
Preferred pilot fuels include diesel fuel and dimethylether. The first and second gaseous fuels may be different fuels but they are preferably the same gaseous fuel. However, the gaseous fuel for one of the stages may be premixed with the pilot fuel so that the pilot fuel and gaseous fuel are introduced together. The first gaseous fuel and the second gaseous fuel are preferably selected from the group consisting of natural gas, liquefied petroleum gas, bio-gas, landfill gas, and hydrogen gas.
Instead of employing a pilot fuel, the engine may be equipped with a spark plug or glow plug to initiate combustion of the gaseous fuel.
In a preferred method of introducing fuel into a cylinder of an operating internal combustion engine having a piston disposed within the cylinder, the fuel comprises a main fuel and a pilot fuel that is auto-ignitable to a degree greater than the main fuel. The method comprises introducing fuel into the cylinder in three stages, whereby,
(a) a first portion of the main fuel is introduced in a first main fuel stage, timed such that the first portion has sufficient time to mix with intake air so that the first portion burns in a pre-mixed combustion mode;
(b) the pilot fuel is introduced in a pilot stage, such that the pilot fuel auto-ignites when the piston is at or near top dead center; and
(c) a second portion of the main fuel is introduced in a second main fuel stage, such that the second portion burns in a diffusion combustion mode;
wherein the quantity of the first portion of main fuel is controlled to provide a main fuel to air ratio during a compression stroke that is less than a calibrated knocking limit.
In this preferred method the second portion may be introduced in a plurality of injection pulses, with the first of the plurality of injection pulses being timed to ignite with the pilot fuel to assist with the ignition of the first portion of the main fuel. That is, part of the second portion of the main fuel may be ignited to assist with the combustion of the first portion of the main fuel.
The first portion of the main fuel is preferably introduced through an auxiliary injection valve into an air induction passage upstream from the cylinder.
The pilot stage preferably begins during a compression stroke. More specifically, the pilot stage preferably begins when the piston is between 120 and 20 crank angle degrees before top dead center so that the pilot fuel has time to form a substantially lean stratified charge prior to combustion.
The second main fuel stage preferably begins when the piston is at or near top dead centre of a compression stroke.
An apparatus is provided for introducing fuel into the cylinder of an operating internal combustion engine having at least one cylinder with a piston disposed therein. The fuel comprises a main fuel and a pilot fuel that is auto-ignitable to a degree greater than the main fuel. The apparatus comprises:
(a) measuring devices for collecting operational data from the engine, the measuring devices comprising a tachometer for measuring engine speed and a sensor for determining throttle position;
(b) an electronic control unit that receives the operational data and processes the data to compute a set of load conditions, the electronic control unit comprising memory for storing control sets of load conditions and predetermined operating modes for the control sets of load conditions, the electronic control unit matching the computed set of load conditions with the control sets of load conditions to select one of a plurality of predetermined operating modes;
(c) a main fuel injection valve controlled by the electronic control unit to introduce the main fuel into the cylinder at times and in quantities determined by the electronic control unit in accordance with the predetermined operating modes and the set of load conditions; and
(d) a pilot fuel injection valve controlled by the electronic control unit to introduce the pilot fuel into the cylinder at times and in quantities determined by the electronic control unit with the predetermined operating modes and the set of load conditions;
wherein the predetermined operating mode comprises a three stage introduction of fuel into the cylinder, whereby a first portion of the main fuel is introduced in a first stage, the pilot fuel is introduced in a second stage and a second portion of the main fuel is introduced in a third stage.
The main fuel injection valve and the pilot fuel injection valve are preferably integrated into a dual fuel injection valve that is operable to inject each one of the main fuel and the pilot fuel independently from the other. An advantage of a dual fuel injection valve is that fewer modifications are required to conventional diesel engines that only have a single fuel injector.
The apparatus may further comprise an auxiliary injection valve associated with an air induction system for introducing the main fuel into an air induction passage. By using an auxiliary injection valve the main fuel may mix with intake air prior to being introduced to the cylinder. The air induction passage may be, for example, an air induction manifold, and is preferably upstream from a turbocharger or supercharger. When the auxiliary injection valve is located in the engine intake port, and the engine comprises a plurality of cylinders, an auxiliary injection valve is provided for each one of the cylinders.
Further advantages for the present dual fuel injection technique will become apparent when considering the drawings in conjunction with the detailed description.